Update Design-Documents.md (flutter#151472)

Make it more clear that every design doc has a corresponding github tracking issue with appropriate label.

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

docs/contributing/Design-Documents.md

@@ -2,6 +2,8 @@ If you want to write a design doc for people to review, we recommend using Googl
 We have a template you can use, at [flutter.dev/go/template](https://flutter.dev/go/template). It describes the process for minting a `flutter.dev/go/foo` shortlink for your design doc.
 We recommend you use that template so that people can immediately recognize that this is a Flutter design document and that it is shared publicly.
 
+After you have created your design doc, the next step is to create a tracking GitHub issue for it. File a new issue to track the design doc using [the design doc issue template](https://github.com/flutter/flutter/issues/new?template=8_design_doc.yml). Assign it to yourself. Add the label "design doc" to the issue.
+
 Don't forget to configure your document's Sharing settings so that everyone has comment access. The idea of sharing the document in this way is not necessarily to proactively obtain feedback from the entire community; it is to make it _possible_ for people to share the document with anyone in the community, whether they work for your employer or not, and whether you have personally shared the document with them yet or not.
 
 The template discusses how to create a shortlink for your design doc (flutter.dev/go/...). When creating the shortlink, remember to test the URL you are publishing in an incognito window!
@@ -33,8 +35,6 @@ If you wish to get feedback on your design doc, you have many options for doing
 
 * If there is an issue already filed on the topic, definitely put a link to the design doc there. People who have found the issue and want to get updates on the topic will have subscribed to the issue, so this is the most effective way to communicate with them.
 
-* File a new issue to track the design doc using [the design doc issue template](https://github.com/flutter/flutter/issues/new?template=8_design_doc.yml). Assign it to yourself.
-
 * Post the link on Discord. You can post it to #hidden-chat to just get feedback from team members. You can post it to one or more of the #hackers-* channels if you want feedback from people who are interested in the general area. You can post it to the global #hackers channel if you want feedback from anyone interested in working on Flutter. If you really want feedback, you can post a request to #announcements and publish it to any server that is following ours.
 
 * If you want feedback from the broad community, tweet out the link and let other team members know so that we can retweet it. Similarly, you can post the request to one of the Flutter reddit channels, such as r/FlutterDev.
@@ -69,4 +69,4 @@ The easiest way to capture videos for design docs is using macOS. Press Command+
 
 ### Diagrams
 
-As we use Google Docs for the text portion of design docs, the easiest way to draw diagrams is using Google Diagrams. Select `Insert` > `Drawing` > `New` to create a new diagram.
+As we use Google Docs for the text portion of design docs, the easiest way to draw diagrams is using Google Diagrams. Select `Insert` > `Drawing` > `New` to create a new diagram.